This invention relates generally to electroluminescent lamps and particularly to such lamps covered by a barrier to protect the active components of the lamp.
Electroluminescent lamps typically have a phosphor particle-containing layer disposed between corresponding electrodes adapted to apply an excitation potential across the phosphor layer to cause light to be emitted. At least one of the electrodes is light transmissive, or at least translucent, to allow escape of the emitted light.
Phosphors are known to deteriorate rapidly in the presence of moisture. Thus, it has been common practice to encapsulate the electroluminescent lamp within a barrier package to impair permeation of moisture into the lamp to delay deterioration of the phosphors for longer lamp life.
Typically the barrier film materials have been copolymers of poly(chlorotrifluoroethylene) (PCTFE), sold under the tradenames "ACLAR" (films), by Allied Chemical Company, and "Kel-F" (resins) by 3M Company, e.g. as taught by Blazek U.S. Pat. No. 3,110,836, Butler et al. U.S. Pat. No. 3,161,797 and Beswick U.S. Pat. No. 3,430,088. PCTFE films, as used, have significant permeability to moisture, and merely delay deterioration of the phosphors for a finite, relatively short period of time.
Increasing crystallinity, e.g. of PCTFE, is known to decrease permeability to water vapor since transport is believed to occur in the amorphous regions of polymers. Thus, the more crystalline, homopolymer PCTFE has better barrier properties than the copolymers or terpolymers. However, the more crystalline PCTFE films are also more susceptible to stress cracking and are less light transparent. Hence, in practice the less effective (more amorphous) copolymers or terpolymers are used even though it is not practical to compensate for reduced barrier properties (moisture permeation up to 100-fold higher) by increasing thickness. Fleming U.S. Pat. No. 4,104,555 sought to retard stress cracking by use of additional layers of encapsulant about the PCTFE.
It is known that diffusion coefficients increase by several orders of magnitude when the temperature is raised above the glass transition temperature (T.sub.g). (Kelleher and Boyle, Modern Plastics Nov. 1979, p. 82.) Hence, prior art PCTFE barrier films are often inadequate, especially at temperatures above the T.sub.g of the PCTFE (e.g. 65.degree. C.), and, at high humidities (e.g. saturated air), even at room temperature.
To compensate for the inadequate moisture barrier, others have sought to extend lamp life by inserting a separate layer of dessicant film such as nylon 6, a poly(amide) condensation product of caprolactam, between the lamp and the outer encapsulant layer, to absorb any moisture that does permeate through the encapsulant layer. Examples are Devol et al. U.S. Pat. No. 3,148,229; Dell U.S. Pat. No. 3,346,758 and Fleming U.S. Pat. No. 4,104,555, which suggest laying a dessicant layer upon the lamp surface, without bonding, within a PCTFE primary encapsulant layer.
Still another approach, Kamijo et al. U.S. Pat. No. 4,455,324, uses a vulcanized fluororubber for the lamp body to seek to provide improved moisture resistance while also using a PCTFE protective outer film.